Petroleum oil and natural gas produced from oil fields and gas fields contain associated gas. The associated gas contains corrosive gas such as carbon dioxide gas (CO2) and hydrogen sulfide (H2S). Line pipes transport the associated gas while transmitting the petroleum oil and the natural gas. Hence, the line pipes suffer from problems of stress corrosion cracking (SCC), sulfide stress corrosion cracking (sulfide stress cracking: SSC), and general corrosion cracking that causes a decrease in wall thickness. Accordingly, stainless steel for the line pipes is required to have an excellent corrosion resistance. Duplex stainless steel has an excellent corrosion resistance. Hence, the duplex stainless steel is used for the line pipes.
The duplex stainless steel for the line pipes is further required to have an excellent yield strength and toughness, in addition to the above-mentioned corrosion resistance. Techniques intended to improve the strength and toughness of duplex stainless steel are disclosed in JP 10-60598A, JP 10-60526A, JP 7-268552A, JP 6-184699A, JP 6-145903A, JP 2726591B, and JP 3155431B.
The duplex stainless steel disclosed in JP 10-60598A and JP 10-60526A contains 2 to 6% of Mo and 4 to 10% of W, and further contains 1 to 4% of Cu. JP 10-60598A and JP 10-60526A describe that aging heat treatment performed on the duplex stainless steel for 4 hours at 480° C. can provide the duplex stainless steel with an excellent strength.
The duplex stainless cast steel disclosed in JP 7-268552A contains 0.1 to 2% of C and 2% or less of Cu. JP 7-268552A describes that precipitation hardening heat treatment performed on the duplex stainless cast steel at 600 to 700° C. can provide the duplex stainless cast steel with a high strength.
The duplex stainless steel disclosed in JP 6-184699A is made of a casting material. The duplex stainless steel contains 0.5 to 4% of Cu and 0.5 to 3% of W. Precipitation hardening heat treatment performed on the duplex stainless steel at 600 to 700° C. can cause fine Nb carbo-nitrides and V carbo-nitrides to disperse therein. JP 6-184699A describes that this can provide the duplex stainless steel with a high strength.
The duplex stainless steel disclosed in JP 6-145903A is made of a casting material. The duplex stainless steel contains 0.5 to 4% of Cu, 0.5 to 3% of W, and 0.1 to 0.5% of Ta. Cu and W are dissolved in ferrite, and strengthen the ferrite. Ta forms carbides, finely disperses in ferrite, and increases the strength thereof. JP 6-145903A describes that the duplex stainless steel can thus be provided with an excellent corrosion fatigue strength.
The duplex stainless steel disclosed in JP 2726591B contains 1 to 4% of Cu and 2% or less of W. Precipitation strengthening treatment performed on the duplex stainless steel at 600 to 700° C. can cause Cu to be precipitated for precipitation strengthening. JP 2726591B describes that the duplex stainless steel can thus be provided with an excellent strength.
The duplex stainless cast member disclosed in JP 3155431B contains 2.6 to 3.5% of Cu, and aging heat treatment is performed thereon for 4 hours at 480° C. JP 3155431B describes that the strength of the steel is improved by precipitation strengthening of Cu.